For wireless communication products, the major components generally include transmitter and receiver. The innovation of the present invention focuses on receiver. For modern TV systems, the architecture of a tuner is divided into two types. One is the architecture with intermediate-frequency (IF) output signal applied in the conventional CAN tuner; the other is the zero-IF architecture widely used in the silicon tuner. Thereby, the apparatus for frequency conversion in the demodulation circuit of the tuner backend has to support both of the input signal types, namely, the IF input signal and the baseband input signal. FIG. 1 shows a block diagram of an apparatus for frequency conversion with IF input signal according to the prior art. As shown in the figure, the apparatus for frequency conversion 10′ according to the prior art comprises an analog-to-digital (A/D) converter 12′, a first multiplier 14′ a second multiplier 16′, an oscillator 18′, a first filer 24′, and a second filter 26′. The A/D converter 12′ receives the IF signal output by the tuner 1′, and converts the IF signal according to a sampling frequency fs to produce a digital signal and transmit it to the first and second multipliers 14′, 16′. The first and second multipliers 14′, 16′ multiply the digital signal by a cosine and a sine signal, respectively, to down-convert the IF signal to a baseband signal, which is transmitted to the first and second filters 24′, 26′ to produce I and Q signals for subsequent circuits.
Because the demodulation circuit has to support both of the IF and baseband signals output by the tuner 1′, the demodulation circuit needs to adopt either the apparatus for frequency conversion 10′ in FIG. 1 or a frequency converter capable of receiving a baseband signal. No matter which type of the apparatus for frequency conversion is adopted, two filters are needed, which occupies the area of the demodulation circuit and increases the cost.